High-strength porous silicon nitride body and process for producing the same

ABSTRACT

A high-porosity and high-strength porous silicon nitride body comprises columnar silicon nitride grains and an oxide bond phase containing 2 to 15 wt. %, in terms of oxide based on silicon nitride, of at least one rare earth element, and has an SiO 2  /(SiO 2  +rare earth element oxide) weight ratio of 0.012 to 0.65 and an average pore size of at most 3.5 μm. The porous silicon nitride body is produced by compacting comprising a silicon nitride powder, 2 to 15 wt. %, in terms of oxide based on silicon nitride, of at least one rare earth element, and an organic binder while controlling the oxygen content and carbon content of said compact; and sintering said compact in an atmosphere comprising nitrogen at 1,650° to 2,200° C. to obtain a porous body having a three-dimensionally entangled structure made up of columnar silicon nitride grains and an oxide bond phase, and having an SiO 2  /(SiO 2  +rare earth element oxide) weight ratio of 0.012 to 0.65.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a porous silicon nitride body having ahigh porosity, a high strength and an excellent machinability, and aprocess for producing the same.

2. Description of the Prior Art

In order to cope with recent environmental issues with car exhaust gasand the like, there has been an increasing demand for a porous ceramicusable as a variety of filters, catalyst carriers and structuralmaterials having a high heat resistance, a high strength and a highthermal shock resistance. For example, there have been desired a filterand a catalyst carrier for removal of CO₂, NO_(x) and black smoke in carexhaust gas, and lightweight car parts for an improvement in respect ofenergy consumption.

A porous silicon nitride body has been proposed as a promising suitablematerial of this kind (PCT International Publication No. WO 94/27929).This porous silicon nitride body is a porous body having a high porosityof at least 30 vol. % as well as a high strength, a high toughness, ahigh thermal shock resistance and a high chemical resistance, whereinβ-Si₃ N₄ grains are mutually bonded with a bond phase comprising atleast one compound of a rare earth element (i.e., Sc, Y or lanthanideelements) in such a way as to have a three-dimensionally entangledstructure.

This porous silicon nitride body is obtained by mixing an Si₃ N₄ powderwith a rare earth element oxide as a sintering aid, compacting theresulting mixture, and then sintering the resulting compact in anatmosphere of pressurized nitrogen. The rare earth element oxide forms aliquid phase together with SiO₂ present on the surfaces of the Si₃ N₄powder through a eutectic reaction at a high temperature duringsintering to melt part of the Si₃ N₄ powder to thereby serve toprecipitate columnar Si₃ N₄ grains. This liquid phase exists as a glassphase or crystalline phase in grain boundaries after sintering, andstrongly bonds the columnar Si₃ N₄ grains to contribute to developmentof the high strength and high toughness properties of the porous Si₃ N₄body. Additionally stated, Y₂ O₃ is cheapest and hence easily availableamong rare earth element oxides.

Sintering of an Si₃ N₄ ceramic is usually effected under a suitable gaspressure applied thereto in order to prevent sublimation of Si₃ N₄ at ahigh temperature. In the foregoing method of PCT InternationalPublication No. WO 94/27929 as well, a gas pressure is applied likewise.For example, the higher the temperature, the higher the pressurenecessary for preventing the sublimation. Accordingly, a maximumpressure of 10 atm is applied up to 1,900° C., a maximum pressure of 40atm is applied at 2,000° C., and a maximum pressure of 100 atm isapplied at 2,100° C., whereby the sublimation is prevented. Thus, in theforegoing method of PCT WO 94/27929, application of the gas pressureduring sintering is aimed only at preventing the sublimation of Si₃ N₄at a high temperature.

Meanwhile, a dense ceramic usable as a general structural member,examples of which include alumina, silicon nitride and zirconia, is hardto work, whereas such a porous body is easily machinable and cantherefore be cut and perforated into an arbitrary shape even withoutusing a special tool such as a diamond cutter. This can greatly lowerthe working cost. On the other hand, however, such a conventional porousbody is usually lowered in mechanical strength due to the presence ofpores, and is therefore hard to put into practical use as a structuralmember. Under such circumstances, it has been desired to provide aporous body having an excellent workability as well as such a sufficientstrength as to be put into practical use as a structural member.Additionally stated, the term "machinability" as used herein is intendedto indicate such properties based on the universally accepted idea thata body can be smoothly subjected to working such as cutting, severance,perforation, or channeling with a drill, a saw, a cutting tool, etc.having an edge made of common carbon steel to form an arbitrary shapewithout cracking, chipping, etc. as if common wood is cut.

A mica-glass ceramic manufactured under the trade name of "MACOR" byCorning Glass Works Corp., which is the only material substantially ofthe prior art in an aspect of machinability, is said to have a littlemachinability. Since the substance of this product contains 30 to 40vol. % of KMg₃ AlSi₃ O₁₀ F₂, however, it is still hardly cut with acommon carbon steel tool, and involves easy cracking and softeningdeformation at about 800° C. Thus, this ceramic product rather has aplurality of defects.

Glass ceramic materials further highly functionalized to be improved instrength and machinability have recently been developed, examples ofwhich are disclosed in Japanese Patent Laid-Open No. 63-134554, JapanesePatent Publication No. 1-59231, Japanese Patent Laid-Open No. 62-7649,Japanese Patent Laid-Open No. 3-88744, and Japanese Patent Laid-Open No.5-178641. However, both the ceramics as disclosed in Japanese PatentLaid-Open No. 63-134554 and Japanese Patent Publication No. 1-59231 havea flexural strength of at most 1,000 kg/cm² to be low in strength, andhence cannot be used as structural materials at all.

On the other hand, the glass ceramics as disclosed in Japanese PatentLaid-Open No. 62-7649, Japanese Patent Laid-Open No. 3-88744, andJapanese Patent Laid-Open No. 5-178641 are said to be well strengthenedto a maximum flexural strength of 5,000 kg/cm² and have machinabilitysuch as workability with a drill. Since these glass ceramics are denseceramics having a relative density close to about 100%, however, themachinability thereof, though improved, are not satisfactory at all.Besides, these glass ceramics are very high in cost because the processof producing crystallizable glass is complicated.

Besides the glass ceramics, a porous Si₃ N₄ --BN ceramic improved instrength and workability has been developed (Japanese Patent Laid-OpenNo. 3-141161). This is said to have a porosity of 6 to 15% and aflexural strength of at most 40 kg/mm², and to be machinable with a highspeed steel cutting tool. However, this porous ceramic is still not wellimproved in workability because the porosity thereof is low.

SUMMARY OF THE INVENTION

In view of the foregoing circumstances of the prior art, an object ofthe present invention is to provide a porous silicon nitride body havingan excellent machinability and usable as a lightweight structuralmember, wherein the strength thereof is further improved while keepingthe porosity thereof high, and a process for producing such a poroussilicon nitride body.

In order to attain the foregoing object, the present invention providesa porous silicon nitride body comprising columnar silicon nitride grainsand an oxide bond phase and having a three-dimensionally entangledstructure made up of the columnar silicon nitride grains and the oxidebond phase wherein the oxide bond phase comprises 2 to 15 wt. %, interms of oxide based on silicon nitride, of at least one rare earthelement and the porous silicon nitride body has an SiO₂ /(SiO₂ +rareearth element oxide) weight ratio of 0.012 to 0.65, an average pore sizeof at most 3.5 μm, and porosity x (vol. %) and three-point flexuralstrength y (MPa) satisfying the relationship:

-14.4x+1300≧y≧-4.8x+360 (provided that 68≧x≧30).

A process for producing such a porous silicon nitride body according tothe present invention comprises: compacting a compact comprising asilicon nitride powder, 2 to 15 wt. %, in terms of oxide based onsilicon nitride, of at least one rare earth element, and an organicbinder while controlling the oxygen content and carbon content of thecompact; and sintering the compact in an atmosphere comprising nitrogenat 1,650° to 2,200° C. to obtain a porous body comprising columnarsilicon nitride grains and an oxide bond phase and having athree-dimensionally entangled structure made up of the columnar siliconnitride grains and the oxide bond phase in which the porous body has aSiO₂ /(SiO₂ +rare earth element oxide) weight ratio of 0.012 to 0.65.

In the foregoing process for producing a porous silicon nitride body,the strength properties thereof can be further improved by sinteringunder a gas pressure of at least 50 atm at 1,650° to 2,200° C.Specifically, the porous silicon nitride body thus obtained contains anoxide bond phase comprising 2 to 15 wt. %, in terms of oxide based onsilicon nitride, of at least one rare earth element; and has an SiO₂/(SiO₂ +rare earth element oxide) weight ratio of 0.012 to 0.65, anaverage pore size of at most 3 μm, and porosity x (vol. %) andthree-point flexural strength y (MPa) satisfying the relationship:

-14.4x+1300≧y≧-8.1x+610 (provided that 50≧x≧30)

-14.4x+1300≧y≧-6.5x+530 (provided that 68≧x≧50).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the region satisfying the porosity versusthree-point flexural strength relationship of the numerical formula 1 inthe porous silicon nitride body of the present invention.

FIG. 2 is a graph showing the region satisfying the porosity versusthree-point flexural strength relationship of the numerical formula 2 inthe porous silicon nitride body of the present invention.

FIG. 3 is a model diagram showing a state of necking of silicon nitridegrains in contact sites thereof as attained when the gas pressure is sethigh during sintering.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

It has been found out that control of the SiO₂ /(SiO₂ +rare earthelement oxide) weight ratio in the range of 0.012 to 0.65 in connectionwith SiO₂ and the rare earth element oxide contained in the porous bodyobtained after sintering is effective as a means for further improvingthe strength of a porous Si₃ N₄ body while keeping the porosity thereofhigh. It has also been found out that control of the atmospheric gaspressure at a level of at least 50 atm during sintering can furtherimprove the properties of a porous Si₃ N₄ body obtained through directhot isostatic press (HIP) sintering under such a high gas pressurewithout encapsulation of a compact while using an HIP apparatus inparticular.

In the present invention, the added rare earth element oxide forms aliquid phase together with SiO₂ present on the surfaces of the Si₃ N₄powder through a eutectic reaction at a high temperature to melt part ofthe Si₃ N₄ powder to thereby serve to precipitate columnar Si₃ N₄ grainsas described hereinbefore. In this case, when the SiO₂ /(SiO₂ +rareearth element oxide) weight ratio in connection with SiO₂ and the rareearth element oxide contained in the sintered body is controlled in therange of 0.012 to 0.65, there can be obtained a porous body very high incomprises β-Si₃ N₄ crystals in a well developed hexagonal columnar form.

More specifically, there can be obtained a high-strength porous Si₃ N₄body, the porosity x (vol. %) and three-point flexural strength y (MPa)of which satisfy the relationship of the following numerical formula 1:

Numerical formula 1!

-14.4x+1300≧y≧-4.8x+360 (provided that 68≧x≧30)

Incidentally, the region satisfying the above numerical formula 1 isshown in FIG. 1.

In the present invention, the SiO₂ /(SiO₂ +rare earth element oxide)weight ratio must be controlled in the range of 0.012 to 0.65, and isdesirably controlled in the range of 0.12 to 0.42. Additionally stated,although SiO₂ and the rare earth element oxide are reacted with nitrogenin Si₃ N₄ or in the atmospheric gas during sintering to exist in theform of, for example, Si--N--Y--O system compounds such as YSiO₂ N,YNSiO₂, Y₂ Si₂ O₇ and Y₂ Si₃ N₄ O₃ in the case where the rare earthelement is Y, the value of the SiO₂ /(SiO₂ +rare earth element oxide)weight ratio as set forth herein is a value obtained through conversionbased on the amount of all these compounds containing Si and the rareearth element. More specifically, it is calculated using the values ofSiO₂ content and rare earth element oxide content obtained throughconversion of the Si wt. % value and the rare earth element wt. % value,respectively, which are chemical analysis values of Si and the rareearth element.

When this weight ratio is lower than 0.012, the amount of SiO₂ is smalland the liquid phase formation temperature is therefore so high that adifficulty may be encountered in liquid phase formation, whereby growthof columnar grains hardly occurs. Even if the liquid phase is formed,the viscosity of the liquid phase is so high that silicon nitride ishardly dissolved in the liquid phase with a low migration speed of thedissolved components within the liquid phase to form a so-calledhalf-fired porous body wherein no three-dimensionally entangledstructure is developed. Thus, the resulting porous body, though high inporosity, is low in strength. When this weight ratio exceeds 0.65 theother way around, the proportion of the rare earth element oxide, whichpromotes growth of columnar grains, is so low that columnar grainshardly grow despite formation of a sufficient amount of the liquid phasedue to lowering of the liquid phase formation temperature, with theresult that densification proceeds to a porosity of less than 30 vol. %to provide a low porosity and a low strength.

In order that the resulting columnar crystal grains are in a hexagonalcolumnar form with developed idiomorphism, the SiO₂ /(SiO₂ +rare earthelement oxide) weight ratio is preferably in the range of 0.12 to 0.42.In this case, there can be obtained a porous body further high instrength. The reason for this is not elucidated, but is believed to bethat growth of Si₃ N₄ grains into a hexagonal columnar form withdeveloped idiomorphism increases the mutual friction of columnar crystalgrains to maximize the effect of entanglement thereof.

The value of the SiO₂ /(SiO₂ +rare earth element oxide) weight ratio isdetermined by control of the oxygen content and carbon content of thecompact before sintering if the amount of the added rare earth elementoxide is fixed. SiO₂ in the form of oxide films is present on thesurfaces of the grains of the Si₃ N₄ powder, while an organic bindercontaining carbon as the principal component is added in compacting.Accordingly, the above-mentioned weight ratio is controlled by adjustingthe amount of SiO₂ as an oxygen source and the amount of the organicbinder as the carbon source and varying the preparation conditions ofthe compact before sintering, such as compacting conditions, binderremoval treatment conditions, etc. Alternatively, positive addition ofan SiO₂ powder and/or addition and mixing of a compound convertible intocarbon by heating as a carbon source, e.g., phenol, may be done.

This will be described specifically. When the compact is fired in anatmosphere comprising oxygen (generally in air) to effect binder removaltreatment thereof, carbon is released as CO gas or CO₂ gas to decreasethe carbon content in the compact by that portion, while at the sametime the surfaces of the Si₃ N₄ powder are also oxidized and convertedinto SiO₂. Thus, the SiO₂ content after the binder removal treatment isincreased. As the binder removal treatment temperature is raised, and asthe treatment time is lengthened, the carbon content is decreased, whilethe SiO₂ content is increased. Under the same binder removal conditions,the larger the amount of the organic binder added when compacting, thehigher the carbon content. Additionally stated, when the binder removaltemperature exceeds 1,000° C., the surface oxidation of the Si₃ N₄powder is liable to rapidly proceed. On the other hand, when it is lowerthan 200° C., the binder is hard to remove from the compact unless thetreatment time is lengthened. Thus, the binder removal treatment isdesirably effected in the temperature range of 200° to 1,000° C.

The SiO₂ content can also be varied by the compacting method. Forexample, when extrusion is done using water as a solvent, the surfacesof the Si₃ N₄ powder are oxidized during extrusion to increase the SiO₂content. On the other hand, when dry compacting is done using an alcoholas a solvent, the SiO₂ content can be lowered.

When the compact after the binder removal treatment, which is controlledin SiO₂ content and carbon content by appropriate selection orcombination of the preparation conditions of the compact such ascompacting conditions, binder removal treatment conditions, etc., isheated in nitrogen for sintering, the residual carbon is reacted withSiO₂ on the surfaces of the Si₃ N₄ powder in keeping with heat-up of thecompact to generate CO gas with a decrease in the SiO₂ content thereof(reduction of SiO₂ with carbon). Thus, the SiO₂ /(SiO₂ +rare earthelement oxide) weight ratio in connection with SiO₂ and the rare earthelement oxide in the resulting sintered body is determined bycontrolling the carbon content and oxygen content of the compact beforesintering.

The Si₃ N₄ powder to be used in the present invention is preferablyamorphous or of α-type, although the Si₃ N₄ powder may partially containa β-type one. When the Si₃ N₄ powder is wholly composed of β-typegrains, no columnar grains are formed. Meanwhile, as the grain size ofthe Si₃ N₄ powder is decreased, the pore size is decreased and thestrength is enhanced. When a trace amount of an impurity element such asfor example Al is mixed in the Si₃ N₄ powder to be used or the rareearth element oxide as a sintering aid, part of Si₃ N₄ is sometimesconverted into compounds such as Si₃ Al₂ O₇ N, which however do notparticularly involve troubles.

Meanwhile, yttrium is not only inexpensive and hence easily available,but also greatly effective in improving the strength. Although as themethod of adding the rare earth element, the rare earth element orelements are usually added in the form of oxide powder, use of a rareearth element alkoxide enables it to be more homogeneously mixed withthe Si₃ N₄ powder, whereby a further strengthened porous body can beobtained.

The sintering temperature is suitably 1,650° to 2,200° C. The higher thetemperature, the higher the effect of improving the strength. When thesintering temperature is too high, however, not only does poor economyensue therefrom, but also growth of grains occurs to increase the poresize, whereby the strength is, the other way around, liable to lower.Thus, a sintering temperature of up to 2,000° C. is preferable. On theother hand, when the sintering temperature is lower than 1,650° C., nocolumnar grains are formed.

As for the gas pressure during sintering, application of a pressure notsubstantially allowing Si₃ N₄ to sublime during sintering, for example,a pressure of up to about 10 atm, will usually suffice. The higher thegas pressure, the higher the strength of the resulting porous body.Sintering is preferably effected under a pressure of at least 50 atm toobtain a porous body having a higher strength as well as a highporosity. Such an effect is remarkably exhibited under a pressure of atleast 100 atm in particular. Additionally stated, although an apparatuscalled "Sinter-HIP" can well cope with sintering under a pressure of atmost 100 atm, an HIP apparatus for exclusive use must be used when thepressure exceeds 100 atm. The upper limit of the pressure is generallyintended to be 2,000 atm, which is the upper limit in existing HIPapparatuses.

High gas pressure sintering represented by HIP sintering has heretoforebeen employed for vanishing final pores in production of a denseceramic. More specifically, pores remaining in a ceramic densified to aporosity of at least 95% by primary sintering (under ordinary pressure)are vanished under a high gas pressure during HIP sintering. Bycontrast, it has been found out that a porous body having a very highstrength can be obtained by applying high gas pressure sinteringdirectly to a Si₃ N₄ compact with the foregoing control of the SiO₂content, the carbon content and the rare earth element oxide content asin the present invention. Additionally stated, when HIP sintering isemployed in the present invention, HIP sintering is effected directly ina gas atmosphere without encapsulation of the compact.

When the atmospheric gas pressure is set at 50 atm or higher duringsintering as described above, there can be obtained a higher-strengthporous Si₃ N₄ body, the porosity x (vol. %) and three-point flexuralstrength y (MPa) of which satisfy the following numerical formula 2:

Numerical Formula 2!

-14.4x+1300≧y≧-8.1x+610 (provided that 50≧x≧30)

-14.4x+1300≧y≧-6.5x+530 (provided that 68≧x≧50)

Incidentally, the region satisfying the foregoing numerical formula 2 isshown in FIG. 2. For comparison, black dots in FIG. 2 representporosities and three-point flexural strengths attained according to PCTInternational Publication No. WO 94/27929.

The following explanation will be given to the effect of improving thestrength of the porous body by sintering under a high gas pressure. Itis believed that the high gas pressure during sintering increases theamounts of Si₃ N₄ and nitrogen gas dissolved in the liquid phase andsimultaneously activates the diffusion of the dissolved Si and Ncomponents to advance mutual necking of hexagonal columnar crystalgrains in the contact sites thereof, with the result that a structurehaving the contact sites 2 of columnar grains 1 developed can beobtained to provide a porous Si₃ N₄ body very high in strength.

The porous Si₃ N₄ body of the present invention obtained according tothe foregoing procedure has a structure wherein columnar Si₃ N₄ grainsare three-dimensionally entangled with the oxide bond phase, and whereinthe oxide bond phase contains 2 to 15 wt. %, in terms of oxide based onSi₃ N₄, of at least one rare earth element. When the amount of the bondphase is smaller than 2 wt % in terms of oxide, columnar grains withdeveloped idiomorphism (hexagonal) are not formed to lower the strengthof the porous body. On the other hand, when it exceeds 15 wt. %, theamount of the grain boundary phase component low in strength isincreased to lower the strength of the porous body as well.

Meanwhile, the average pore size of the porous Si₃ N₄ body is at most3.5 μm, preferably at most 3 μm, while the porosity thereof is in therange of 30 to 68 vol. %. When the average pore size exceeds 3.5 μm, thestrength is lowered. The lower limit of the average pore size is notparticularly limited because it is determined by the grain size of theSi₃ N₄ powder as a starting material. However, the lower limit is 0.05μm when a commercially available Si₃ N₄ powder is used, but this doesnot apply to the case where such a powder is specially prepared. Aporous body having a porosity of less than 30 vol. % is hard to produceaccording to the process of the present invention in an aspect of therelationship between the grain growth rate and the densification speed.When the porosity exceeds 68 vol. %, the shape of the compact cannot bemaintained during compacting because the porosity is too high.

Although the foregoing porous Si₃ N₄ body of the present invention showssufficiently high strength properties as a structural member, it is soeasily machinable that it can be smoothly cut, severed, perforated,channeled, etc. into an arbitrary shape with a cutting tool having anedge made of either common carbon steel or alloy steel, e.g., a drill, asaw, or a cutting tool, without cracking, chipping, etc. as if commonwood is cut.

Further, the porous Si₃ N₄ body of the present invention is so low inYoung's modulus for its high strength that it has a feature of excellentimpact absorption. Young's modulus is lowered with an increase inporosity. The porous Si₃ N₄ body of the present invention has a Young'smodulus in the range of 15 GPa (porosity: 68 vol. %) to 100 GPa(porosity: 30 vol. %). This porous Si₃ N₄ body is also so low in thermalconductivity that it can be used as a heat insulating material. Theporous Si₃ N₄ body of the present invention especially satisfies thefollowing relationship between the thermal conductivity z (W/mK) and theporosity x (vol. %):

z≧-0.15x+9.5

Further, when the porosity of the porous Si₃ N₄ body becomes 40 vol. %or higher, the dielectric constant thereof becomes 3.6 or lower. Si₃ N₄is a material having a low dielectric constant (ε) (ε=7.6) among variousceramics. Further, the dielectric constant of a ceramic is lowered asthe porosity thereof is increased. In view of the foregoing, a porousSi₃ N₄ body has been a greatly hoped-for low-dielectric-constant body.The porous Si₃ N₄ body of the present invention is a material havingsuch a very high strength that no conventional low-dielectric-constantmaterials have, and is low in dielectric loss to the extent of noproblem in practical use.

Thus, the porous silicon nitride body of the present invention is amaterial balanced between the porosity and strength thereof at a veryhigh level. Accordingly, when a filter is produced by making the most ofthe features of the porous body, the filter can be set to have a smallthickness, and can exhibit a high permeability performance due to thehigh porosity thereof. Further, the porous body can serve as alightweight high-strength ceramic capable of exhibiting a highperformance as a variety of structural members such as automotive parts.Furthermore, since the porous body is a so-called machinable ceramic,which can be machined freely, it can contribute to a great reduction ofthe working cost, which accounts for a major proportion of theproduction cost of a ceramic part.

Since the porous Si₃ N₄ body of the present invention is also endowedwith a low dielectric constant and high strength properties, it iseffective as a substrate material little in transmission loss in a highfrequency range like that of millimeter waves. Besides, it can beutilized as a high-performance radar transmission material.

Further, when the porous Si₃ N₄ body of the present invention is used asa friction material under oil lubrication conditions in a furtherapplication thereof, the friction coefficient thereof can be expected tobe lowered due to infiltration of oil into pores thereof. Besides, itcan be used as a sound absorbing material for use in road walls and thelike, and as a wall material for use in houses and the like by makingthe most of its merits, i.e., a light weight, a high porosity, a highstrength and a low thermal conductivity.

The following examples illustrate the present invention morespecifically. The proportions of the compounds of rare earth elementsused in the examples are expressed by weight % in terms of oxides basedon silicon nitride, unless otherwise indicated.

EXAMPLE 1

An α-Si₃ N₄ powder of 0.4 μm in average grain size was blended with a Y₂O₃ powder of 0.015 μm in average grain size or Y(OC₂ H₅)₃ as a Yalkoxide at a proportion as shown in Table 1, and further admixed with15 wt. %, based on the sum of the foregoing powders, of methylcelluloseas an organic binder. The resulting mixed powder was compacted to have arelative density of 44%. Each compact was heated in air at 500° C. for 1hour to effect binder removal treatment thereof, and then fired in anitrogen gas atmosphere under sintering conditions as shown in Table 1to obtain a porous Si₃ H₄ body. Additionally stated, the used Si₃ N₄powder contained 2.25 wt. % of SiO₂ as a surface oxide film.

                  TABLE 1                                                         ______________________________________                                                        Sintering Conditions                                          Sintering Aid     Temp.     Time   Pressure                                   Sample Kind       wt. %   (°C.)                                                                          (hr) (atm)                                  ______________________________________                                         1*    Y.sub.2 O.sub.3                                                                          1.9     1800    2    5                                       2     Y.sub.2 O.sub.3                                                                          2       1800    2    5                                       3     Y.sub.2 O.sub.3                                                                          3       1800    2    1000                                    4     Y.sub.2 O.sub.3                                                                          4.5     1800    2    5                                       5     Y.sub.2 O.sub.3                                                                          4.5     1800    2    49                                      6     Y.sub.2 O.sub.3                                                                          4.5     1800    2    51                                      7     Y.sub.2 O.sub.3                                                                          4.5     1800    2    120                                     8     Y.sub.2 O.sub.3                                                                          4.5     1800    2    1000                                    9     Y.sub.2 O.sub.3                                                                          8       1800    2    2000                                   10     Y.sub.2 O.sub.3                                                                          15      1800    2    2000                                   11     Y.sub.2 O.sub.3                                                                          16.7    1800    2    2000                                   12     Y(OC.sub.2 H.sub.5).sub.3                                                                8       2000    5    1000                                   13     Y(OC.sub.2 H.sub.5).sub.3                                                                8       2000    7    1000                                   14     Y(OC.sub.2 H.sub.5).sub.3                                                                8       2000    8    2000                                   ______________________________________                                         (Note) The sample with * in the table is of Comparative Example.         

The following experiments were carried out for each porous Si₃ N₄ bodysample thus obtained.

(1) SiO₂ /(SiO₂ +rare earth element oxide) weight ratio: This wasexamined by chemical analysis of the porous Si₃ N₄ body.

(2) Porosity and Average Pore Size: They were measured with a mercuryporosimeter.

(3) Flexural Strength: The flexural strength at room temperature wasmeasured by a three-point flexural strength test in accordance with JIS1601.

(4) Young's Modulus: This was calculated from a stress-strain curveobtained in the flexural strength test.

(5) Fuel Consumption: The porous Si₃ N₄ body was worked into a tappetshim of 30 mm in diameter and 5 mm in thickness, which was thenmirror-polished to a degree of surface roughness Ra=0.01 μm, thenassembled with a steel cam shaft, and installed in a gasoline engine carof 1,500 cc displacement, followed by examination of the 10 mode fuelconsumption thereof.

The results are shown in the following Table 2. For comparison, the fuelconsumption was examined using a dense Si₃ N₄ (strength: 1,500 MPa,specific gravity: 3.24) as well as steel tappet shim in the same manneras described above. The results are also shown in Table 2.

                                      TABLE 2                                     __________________________________________________________________________         SiO.sub.2 /(SiO.sub.2 +                                                                        Young's                                                                            Flexural                                                                           Fuel                                               rare earth                                                                           Porosity                                                                           Pore Size                                                                          Modulus                                                                            Strength                                                                           Consumption                                   Sample                                                                             element oxide)                                                                       (%)  (μm)                                                                            (GPa)                                                                              (MPa)                                                                              (km/l)                                        __________________________________________________________________________     1*  0.54   54   1.4  22   77   --                                             2   0.53   54   1.4  25   120  --                                             3   0.43   51   1.3  39   200  --                                             4   0.33   50   1.4  38   133  --                                             5   0.33   50   1.4  39   188  --                                             6   0.33   48   1.4  46   288  --                                             7   0.33   45   1.4  57   299  --                                             8   0.33   40   1.2  65   422  --                                             9   0.22   38   1.1  70   533  18.4                                          10   0.13   38   1.1  71   399  --                                            11   0.119  39   1.1  70   333  --                                            12   0.22   32   0.5  77   630  17.3                                          13   0.22   31   0.5  78   634  17.1                                          14   0.22   30   0.4  89   644  17.2                                          15*  dense Si.sub.3 N.sub.4 tappet shim                                                                       16.4                                          16*  steel tappet shim          15.9                                          __________________________________________________________________________     (Note) The samples with * in the table are of Comparative Example.       

It is understandable from the above results that the porous Si₃ N₄ bodyof the present invention can keep the porosity thereof high and has avery high three-point flexural strength for its porosity. It is alsounderstandable that the porous Si₃ N₄ body of the present invention cangreatly improve the fuel consumption efficiency of a car engine when itis used as a tappet shim.

EXAMPLE 2

Substantially the same procedure as in Example 1 was repeated to providecompacts having a relative density of 44% except for use of an α-Si₃ N₄powder of 3.0 μm in average grain size and a variety of rare earthelement compounds of 0.005 μm in average grain size instead of Y₂ O₃ asa rare earth element compound. Each compact thus obtained was subjectedto binder removal treatment in air at 450° C. for 1.5 hours, and thensintered under a pressure of 1,000 to 2,100 atm at a temperature of1,600° to 1,990° C. for 2 hours as shown in Table 3 to form a porous Si₃N₄ body. Additionally stated, the used Si₃ N₄ powder contained 3.25 wt.% of SiO₂.

                  TABLE 3                                                         ______________________________________                                                       Sintering Conditions                                                  Sintering Aid                                                                           Temp.     Time   Pressure                                    Sample   Kind     wt. %  (°C.)                                                                          (hr) (atm)                                   ______________________________________                                        17*      CeO.sub.2                                                                              8      1600    2    1000                                    18       CeO.sub.2                                                                              8      1650    2    1000                                    19       CeO.sub.2                                                                              8      1750    8    1000                                    20       CeO.sub.2                                                                              8      1750    6    1000                                    21       CeO.sub.2                                                                              8      1750    4    1000                                    22       CeO.sub.2                                                                              8      1750    2.5  1000                                    23       CeO.sub.2                                                                              8      1990    2    1000                                    24       Nd.sub.2 O.sub.3                                                                       8      1990    2    1000                                    25       Gd.sub.2 O.sub.3                                                                       8      1990    2    1000                                    26       Dy.sub.2 O.sub.3                                                                       8      1990    2    1000                                    27       Yb.sub.2 O.sub.3                                                                       8      1990    2    2100                                    28       Yb.sub.2 O.sub.3                                                                       8      1990    5    2000                                    ______________________________________                                         (Note) The sample with * in the table is of Comparative Example.         

Each porous Si₃ N₄ body sample thus obtained was evaluated according tothe same experiments as described hereinabove. The results are shown inTable 4.

                  TABLE 4                                                         ______________________________________                                              SiO.sub.2 /(SiO.sub.2 +                                                                           Pore   Young's                                                                              Flexural                                    rare earth  Porosity                                                                              Size   Modulus                                                                              Strength                              Sample                                                                              element oxide)                                                                            (%)     (μm)                                                                              (GPa)  (MPa)                                 ______________________________________                                        17*   0.29        55      2.9    27     44                                    18    0.29        50      2.7    40     236                                   19    0.29        44      3.5    48     200                                   20    0.29        44      3.2    47     200                                   21    0.29        44      3.0    47     290                                   22    0.29        44      2.2    48     415                                   23    0.29        32      1.8    95     607                                   24    0.29        32      1.8    81     609                                   25    0.29        31      1.8    88     599                                   26    0.29        31      1.8    88     588                                   27    0.29        31      1.8    88     596                                   28    0.29        31      1.8    99     633                                   ______________________________________                                         (Note) The sample with * in the table is of Comparative Example.         

EXAMPLE 3

Substantially the same procedure as in Example 1 was repeated to providecompacts having a relative density of 30%, 50% or 75% except for use ofan α-Si₃ N₄ powder of 0.05 μm in average grain size and Er₂ O₃ of 0.005μm in average grain size instead of Y₂ O₃ as a rare earth elementcompound. Each compact thus obtained was subjected to binder removaltreatment in air at 600° C. for 1 hour, and then sintered under apressure of 5 to 1,000 atm at a temperature of 1,850° to 2,200° C. for 2to 2.5 hours as shown in Table 5 to form a porous Si₃ N₄ body.Additionally stated, the used Si₃ N₄ powder contained 3.25 wt. % ofSiO₂. Samples were also prepared in the same manner as described aboveexcept that phenol in an amount of 0.4 to 0.8 wt. % based on the weightof the Si₃ N₄ powder was further added a carbon source other than theorganic binder.

                  TABLE 5                                                         ______________________________________                                                     Compact                                                                              Sintering Conditions                                            Er.sub.2 O.sub.3                                                                      Phenol   Density                                                                              Temp.  Time Pressure                            Sample                                                                              (wt. %) (wt. %)  (%)    (°C.)                                                                         (hr) (atm)                               ______________________________________                                        29*   1.7     not added                                                                              30     1850   2    5                                   30    2       not added                                                                              30     1850   2    5                                   31    2       not added                                                                              30     1850   2    1000                                32    4       not added                                                                              30     1850   2    5                                   33    8       not added                                                                              30     1850   2    5                                   34    8       not added                                                                              30     1850   2    55                                  35    8       not added                                                                              30     1850   2    1000                                36    8       not added                                                                              50     1850   2    1000                                37    8       not added                                                                              70     1850   2.5  50                                  38    8       not added                                                                              70     1850   2.5  1000                                39    15      not added                                                                              30     1850   2    5                                   40    15      0.4      30     1850   2    5                                   41    15      0.5      30     1950   2    5                                   42    15      0.6      30     1950   2    5                                   43    15      0.6      30     2200   2    56                                  44    15      0.6      30     2200   2    1000                                45*   15.5    0.8      30     2200   2    1000                                46*   16.5    0.8      30     2200   2    5                                   ______________________________________                                         (Note) The samples with * in the table are of Comparative Example.       

Each porous Si₃ N₄ body thus obtained was evaluated according to thesame experiments as described hereinabove. The results are shown inTable 6.

                  TABLE 6                                                         ______________________________________                                                 SiO.sub.2 /(SiO.sub.2 +                                                                            Pore  Flexural                                           rare earth Porosity  Size  Strength                                  Sample   element oxide)                                                                           (%)       (μm)                                                                             (MPa)                                     ______________________________________                                        29*      0.625      69        0.02  66                                        30       0.62       65        0.06  100                                       31       0.62       68        0.06  310                                       32       0.45       59        0.07  111                                       33       0.29       58        0.07  122                                       34       0.29       58        0.07  199                                       35       0.29       58        0.07  400                                       36       0.29       50        0.06  550                                       37       0.29       32        0.05  400                                       38       0.29       30        0.07  850                                       39       0.18       57        0.08  123                                       40       0.08       59        0.07  129                                       41       0.08       60        0.07  100                                       42       0.012      67        0.07  101                                       43       0.012      67        0.07  88                                        44       0.012      67        0.07  94                                        45*      0.010      67        0.07  53                                        46*      0.009      69        0.07  47                                        ______________________________________                                         (Note) The samples with * in the table are of Comparative Example.       

Regarding Samples 32 to 35 in the above Table 5, the pure waterpermeability performance of each porous Si₃ N₄ body was measured. In themeasurement, the porous body was formed into a flat plate of 25 mm indiameter and 0.1 mm in thickness, with which direct filtration was doneunder a feed pressure of 5 atm (atmospheric pressure on the permeate'sside) to make the measurement. As a result, Samples 32 and 33 producedunder a pressure of 5 atm during sintering were broken in the course offiltration, whereas Samples 34 and 35 produced under a pressure of atleast 50 atm during sintering were capable of filtration withoutbreakage. The permeate flow rate during filtration was 6.8 ml/min/cm²/atm in the case of Sample 34 and 7.0 ml/min/cm² /atm in the case ofSample 35.

EXAMPLE 4

An α-Si₃ N₄ powder of 0.13 μm in average grain size was admixed with aY₂ O₃ powder of 0.3 μm in average grain size at a proportion as shown inTable 7, and further admixed with 12 wt. %, based on the whole ceramicpowder, of a polyethylene glycol binder as an organic binder, followedby compacting to a compact density of 50%. Each of the resultingcompacts was heated in air at 320° C. for 1 hour to effect binderremoval treatment thereof, and then sintered in a nitrogen gasatmosphere under a pressure as shown in Table 7 at 1,800° C. for 2 hoursto obtain a porous Si₃ N₄ body. Additionally stated, the used Si₃ N₄powder contained 3.3 wt. % of SiO₂.

Each porous Si₃ N₄ body sample thus obtained was evaluated according tothe same experiments as described hereinabove as well as the followingexperiments. The results are shown in Table 7 to 9.

(6) Average Aspect Ratio: The major axes and minor axes of 50 grainsarbitrarily chosen from a scanning electron microscope photomicrographfor each sample were measured to calculate the respective average valuesthereof and the average aspect ratio (average major axis/average minoraxis) was obtained.

(7) Measurement of Thermal Conductivity: This was measured using a testpiece of 10 mm in diameter and 1 mm in thickness according to the laserflash method.

(8) Machinability: A high speed steel cutting tool was installed in alathe, with which a surface of a test piece of 100 mm in diameter cutout from each sample was shaved through a single feed motion at arevolution speed of 800 rpm to a cut depth of 1 mm and a cut length of100 mm. The machinability was evaluated according to the three ratings:i.e., ∘ for a difference of at most 0.01 mm in outer diameter betweenboth ends, ▪ for a difference of more than 0.01 mm to 0.02 mm in outerdiameter therebetween, and x for a difference of more than 0.02 mm inouter diameter therebetween.

                                      TABLE 7                                     __________________________________________________________________________                               Average Crystal                                            Sintering       Pore                                                                             Grain Size                                             Y.sub.2 O.sub.3                                                                   Pressure                                                                           SiO.sub.2 /                                                                          Porosity                                                                          Size                                                                             Major                                                                              Minor                                                                              Aspect                                   Sample                                                                            (wt. %)                                                                           (atm)                                                                              (SiO.sub.2 + Y.sub.2 O.sub.3)                                                        (%) (μm)                                                                          Axis (μm)                                                                       Axis (μm)                                                                       Ratio                                    __________________________________________________________________________    47* 0.5 3    0.77   45  0.8                                                                               1   0.5   2                                       48*  1  3    0.71   39  1.5                                                                               3   0.8   3.75                                    49*  2  3    0.66   48  1.8                                                                              4.2  0.9   4.67                                    50   4  3    0.42   48  0.8                                                                              15   1    15                                       51   8  60   0.22   58  3.5                                                                              20   1.5  13.3                                     52  12  63   0.19   57  3.0                                                                              20   1.6  12.5                                     53  15  7    0.12   55  4.0                                                                              18   1.8  10                                       54* 17  3    0.14   49  3.2                                                                              15   1.9   7.89                                    55* 20  3    0.04   50  3.0                                                                              25   2.0  12.5                                     __________________________________________________________________________     (Note) The samples with * in the table are of Comparative Example.       

                  TABLE 8                                                         ______________________________________                                                Flexural    Thermal                                                           Strength    Conductivity                                              Sample  (MPa)       λ(W/mK)                                                                          Machinability                                   ______________________________________                                        47*     40          5.0       x                                               48*     88          4.4       x                                               49*     99          3.2       x                                               50      230         2.7       ∘                                   51      188         1.6       ∘                                   52      195         1.7       ∘                                   53      166         1.5       ∘                                   54*     95          1.6       ∘                                   55*     55          0.3       ▪                                     ______________________________________                                         (Note) The samples with * in the table are of Comparative Example.       

Further, a starting Si₃ N₄ material powder of 10 μm in average grainsize, a starting BN material powder of 10 μm in average grain size, anda starting SiC material powder of 1 μm in average grain size were mixedwith a Y₂ O₃ powder of 4 μm in average grain size and an Al₂ O₃ powderof 2 μm in average grain size as sintering aids at a proportion, basedon the combined weight of the Si₃ N₄, BN and SiC powders as shown inTable 9 and further admixed with 12 wt. %, based on all the foregoingpowders, of a polyethylene glycol binder as an organic binder. Theresulting mixture was compacted, and then sintered in a nitrogen gasatmosphere under a pressure of 160 kg/cm² at 1,800° C. for 1 hour. Thus,5 kinds of Si₃ N₄ --BN composite materials were obtained. These samplesof Comparative Example were evaluated in the same manner as describedabove. The results are shown in Table 9.

                                      TABLE 9                                     __________________________________________________________________________                    Sintering Aid                                                                            Flexural                                           Si.sub.3 N.sub.4                                                                      BN  SiC (wt. %)                                                                              Porosity                                                                          Strength                                           Sample                                                                            (wt. %)                                                                           (wt. %)                                                                           (wt. %)                                                                           Y.sub.2 O.sub.3                                                                  Al.sub.2 O.sub.3                                                                  (%) (MPa)                                                                             Machinability                                  __________________________________________________________________________    56* 70  20  10  9  1    6  400 x                                              57* 25  40  35  2  1   10  200 x                                              58* 45  50   5  6  2   10  230 x                                              59* 20  60  20  0.5                                                                              0.5 12  150 x                                              60* 26  70   4  3  2   15  100 x                                              __________________________________________________________________________     (Note) The samples with * in the table are of Comparative Example.       

EXAMPLE 5

An α-Si₃ N₄ powder of 0.6 μm in average grain size was admixed with 3wt. % of a variety of rare earth element oxide powders of 0.4 μm inaverage grain size as shown in the following Table 10 as a sinteringaid. The resulting mixed powder was admixed with 2 wt. %, based on thewhole ceramic powder, of methylcellulose as an organic binder, and thencompacted to a compact density of 50%. Each of the resulting compactswas heated in air at 800° C. for 1 hour to effect binder removaltreatment thereof, and then sintered in nitrogen gas under a pressure of500 atm at a temperature as shown in Table 10 for 2 hour to obtain aporous Si₃ N₄ body. Additionally stated, the used Si₃ N₄ powdercontained 1.7 wt. % of SiO₂.

Each porous Si₃ N₄ body sample thus obtained was evaluated according tothe same tests as described hereinabove, and further tested for thefollowing perforation machinability.

(9) Perforation Machinability: A perforation machinability test wascarried out using a cutting tool, i.e., steel drill, at a revolutionspeed of 50/min to make evaluation according to the ratings: ∘ for aperforated sample, ▪ for a chipped sample, and x for a broken sample.The results are shown in Tables 10 and 11. The Si₃ N₄ --BN compositematerials of Comparative Example obtained in Example 4 and a glassceramic produced according to Table 1 in Japanese Patent Laid-Open No.5-178641 were also tested for perforation machinability, but thesecomparative samples were all broken in the course of perforationmachining.

                                      TABLE 10                                    __________________________________________________________________________                               Average Crystal                                            Sintering                                                                          SiO.sub.2 /(SiO.sub.2 +                                                                  Pore                                                                             Grain Size                                                 Temp.                                                                              rare earth                                                                           Porosity                                                                          Size                                                                             Major                                                                              Minor                                                                              Aspect                                   Sample                                                                            Aid (°C.)                                                                       element oxide                                                                        (%) (μm)                                                                          Axis (μm)                                                                       Axis (μm)                                                                       Ratio                                    __________________________________________________________________________     61*                                                                              La.sub.2 O.sub.3                                                                  1600 0.36   48  0.3                                                                              0.9  0.2  4.5                                      62  La.sub.2 O.sub.3                                                                  1700 0.35   48  0.3                                                                              3.8  0.2  19                                       63  La.sub.2 O.sub.3                                                                  1900 0.35   39  0.1                                                                              5.0   0.15                                                                              33                                       64  La.sub.2 O.sub.3                                                                  2200 0.37   30   0.05                                                                            8.0  0.9  8.9                                      65  Ce.sub.2 O.sub.3                                                                  1700 0.37   48  0.2                                                                              3.9  0.3  13                                       66  Nd.sub.2 O.sub.3                                                                  1700 0.40   48  0.2                                                                              4.2   0.25                                                                              16.8                                     67  Gd.sub.2 O.sub.3                                                                  1700 0.35   48  0.2                                                                              3.6  0.3  12                                       68  Dy.sub.2 O.sub.3                                                                  1700 0.35   49  0.3                                                                              3.8  0.2  19                                       69  Yb.sub.2 O.sub.3                                                                  1700 0.37   49  0.2                                                                              3.9  0.3  13                                       __________________________________________________________________________     (Note) The sample with * in the table is of Comparative Example.         

                  TABLE 11                                                        ______________________________________                                                Flexural    Thermal                                                           Strength    Conductivity                                                                            Perforation                                     Sample  (MPa)       λ(W/mK)                                                                          Machinability                                   ______________________________________                                        61*     90          2.7       ▪                                     62      266         3.5       ∘                                   63      177         2.2       ∘                                   64      366         7.4       x                                               65      255         2.8       ∘                                   66      239         2.7       ∘                                   67      241         3.3       ∘                                   68      277         3.2       ∘                                   69      299         2.9       ∘                                   ______________________________________                                         (Note) The sample with * in the table is of Comparative Example.         

EXAMPLE 6

An α-Si₃ N₄ powder of 0.25 μm in average grain size was admixed with aY₂ O₃ powder of 0.02 μm in average grain size, and then admixed with 12wt. %, based on all the ceramic powders, of methylcellulose as anorganic binder. The resulting mixed powder was compacted. Each compactwas heated in air at 500° C. for 1 hour to effect binder removaltreatment thereof, and then sintered in a nitrogen gas atmosphere underconditions as shown in the following Table 12 for 2 hours to obtain aporous Si₃ N₄ body. In Comparative Example, the same procedure asdescribed above was conducted except that 2 wt. % of an Al₂ O₃ powder of0.03 μm in average grain size and 5 wt. % of a Y₂ O₃ powder of 0.02 μmin average grain size were added to the same Si₃ N₄ powder as used aboveand sintering was carried out under conditions as shown in Table 12.Additionally stated, the used Si₃ N₄ powder contained 2.0 wt. % of SiO₂.

                  TABLE 12                                                        ______________________________________                                        Sintering Aid   Compact    Sintering Conditions                                      Kind         Density    Temp.  Pressure                                Sample (wt. %)      (g/cm.sup.3)                                                                             (°C.)                                                                         (atm)                                   ______________________________________                                        70*    Y.sub.2 O.sub.3 (8)                                                                        1.38       1830   4                                       71     Y.sub.2 O.sub.3 (8)                                                                        1.4        1830   5                                       72     Y.sub.2 O.sub.3 (8)                                                                        1.4        1830   50                                      73     Y.sub.2 O.sub.3 (8)                                                                        1.45       1830   52                                      74     Y.sub.2 O.sub.3 (8)                                                                        1.5        1830   60                                      75     Y.sub.2 O.sub.3 (8)                                                                        1.6        1830   5                                       76     Y.sub.2 O.sub.3 (8)                                                                        1.6        1830   66                                      77     Y.sub.2 O.sub.3 (8)                                                                        1.7        1830   50                                      78     Y.sub.2 O.sub.3 (8)                                                                        1.75       1830   120                                     79     Y.sub.2 O.sub.3 (8)                                                                        1.8        1830   150                                     80*    Y.sub.2 O.sub.3 (5) + Al.sub.2 O.sub.3 (2)                                                 1.6        1400   4                                       81*    Y.sub.2 O.sub.3 (5) + Al.sub.2 O.sub.3 (2)                                                 1.6        1500   4                                       82*    Y.sub.2 O.sub.3 (5) + Al.sub.2 O.sub.3 (2)                                                 1.6        1600   4                                       ______________________________________                                         (Note) The samples with * in the table are of Comparative Example.       

Each porous Si₃ N₄ body sample thus obtained was evaluated according tothe same tests as described hereinabove, and further subjected, using aresonator, to measurement of dielectric constant and dielectric loss ata frequency of 1 GHz, which were calculated from a resonance frequency.The results are shown in Table 13.

                                      TABLE 13                                    __________________________________________________________________________        SiO.sub.2 /(SiO.sub.2 +                                                                  Pore    Flexural                                                                          Dielectric                                                                         Dielectric                                        rare earth                                                                           Porosity                                                                          Size                                                                              Aspect                                                                            Strength                                                                          Constant                                                                           Loss                                          Sample                                                                            element oxide                                                                        (%) (μm)                                                                           Ratio                                                                             (MPa)                                                                             ε                                                                          (×10.sup.-3)                            __________________________________________________________________________     70*                                                                              0.20   70  0.5 19   48 2.44 1.00                                          71  0.21   65  0.36                                                                              20   80 2.53 0.99                                          72  0.21   65  0.38                                                                              20  120 2.53 0.98                                          73  0.22   60  0.33                                                                              21  165 2.69 0.96                                          74  0.20   55  0.3 18  200 2.87 0.99                                          75  0.22   50  0.22                                                                              19  170 3.07 0.96                                          76  0.22   50  0.2 20  250 3.07 0.94                                          77  0.22   45  0.15                                                                              17  300 3.30 0.88                                          78  0.22   40  0.11                                                                              16  459 3.57 0.93                                          79  0.20   38  0.1 19  488 3.63 0.88                                           80*                                                                              --     50  0.2  1   45 3.65 1.22                                           81*                                                                              --     45  0.2  1   80 3.91 1.33                                           82*                                                                              --     35  0.15                                                                               1  150 4.56 1.06                                          __________________________________________________________________________     (Note) The samples with * in the table are of Comparative Example.       

According to the present invention, there can be provided an easilymachinable porous silicon nitride body having a very high strengthdespite its high porosity and a lightweight. Further, this poroussilicon nitride body is low in Young's modulus and hence excellent inimpact absorptivity, and low in thermal conductivity and hence excellentin heat insulating properties. Accordingly, the porous silicon nitridebody of the present invention is useful not only as a filter forseparation of a liquid or a gas as a matter of course, but also as aheat insulating material, a sound absorbing material and a variety ofstructural materials for automobile parts and the like. Besides, itenables the working cost to be greatly lowered.

Since the porous silicon nitride body of the present invention also hasa feature of low dielectric constant, it is effective as a radartransmission material, and can be utilized as an electronic partsubstrate material such as a substrate material low in transmission lossat a high frequency like that of millimeter waves.

What is claimed is:
 1. A high-strength porous silicon nitride bodycomprising columnar silicon nitride grains and an oxide bond phase andhaving a three-dimensionally entangled structure made up of saidcolumnar silicon nitride grains and said oxide bond phase wherein saidoxide bond phase comprises 2 to 15 wt. %, in terms of oxide based onsilicon nitride, of at least one rare earth element and said poroussilicon nitride body has an SiO₂ /(SiO₂ +rare earth element oxide)weight ratio of 0.012 to 0.65, an average pore size of at most 3 μm, andporosity x (vol. %) and three-point flexural strength y (MPa) satisfyingthe relationship:-14.4x+1300≧y≧-8.1x+610 (provided that 50≧x≧30)-14.4x+1300≧y≧-6.5x+530 (provided that 68≧x≧50).
 2. A high-strengthporous silicon nitride body as claimed in claim 1, in which said SiO₂/(SiO₂ +rare earth element oxide) weight ratio is 0.12 to 0.42.
 3. Ahigh-strength porous silicon nitride body as claimed in claim 1, inwhich the thermal conductivity z (W/mK) and porosity x (vol. %) thereofsatisfy the relationship:z≧-0.15x+9.5.
 4. A high-strength porous siliconnitride body as claimed in claim 1, in which the Young's modulus thereofis 15 to 100 GPa.
 5. A high-strength porous silicon nitride body asclaimed in claim 1, in which the porosity thereof is 40 to 68 vol. % andthe dielectric constant thereof is at most 3.6.
 6. A high-strengthporous silicon nitride body as claimed in claim 1, in which said poroussilicon nitride body can be cut with a cutting tool made using an alloysteel or carbon steel.
 7. A high-strength porous silicon nitride body asclaimed in claim 1, in which said rare earth element is yttrium.
 8. Aprocess for producing a high-strength porous silicon nitride body ofclaim 2, comprising: compacting a compact comprising a silicon nitridepowder, 2 to 15 wt %, in terms of oxide based on silicon nitride, of atleast one rare earth element, and an organic binder while controllingthe oxygen content and carbon content of said compact; and sinteringsaid compact in an atmosphere comprising nitrogen at 1,650° to 2,220° C.and a pressure of at least 50 atmospheres to obtain a porous bodycomprising columnar silicon nitride grains and an oxide bond phase andhaving a three-dimensionally entangled structure made up of saidcolumnar silicon nitride grains and said oxide bond phase in which saidporous body has an SiO₂ /(SiO₂ +rare earth element oxide) weight ratioof 0.012 to 0.65.
 9. A process for producing a high-strength poroussilicon nitride body as claimed in claim 8, in which said compactcontains an SiO₂ powder as an oxygen source and/or a compoundconvertible into carbon by heating as a carbon source.
 10. A process forproducing a high-strength porous silicon nitride body as claimed inclaim 8, in which the oxygen content and carbon content of said compactare controlled to obtain a porous body having an SiO₂ /(SiO₂ +rare earthelement oxide) weight ratio of 0.12 to 0.42 after said sintering.
 11. Aprocess for producing a high-strength porous silicon nitride body asclaimed in claim 8, in which said rare earth element is yttrium.